Deprecate the -warn-on-potentially-unavailable-enum-case option.

When `-warn-on-potentially-unavailable-enum-case` was introduced, the build
system was required to invoke `swift-frontend` at artificially low deployment
targets when emitting `.swiftinterface` files for legacy architectures. Because
the deployment target was low, some availability diagnostics needed to be
de-fanged in order to allow module interface emission to succeed. Today, the
build system is able to use the correct deployment target when emitting module
interfaces and the `-warn-on-potentially-unavailable-enum-case` is superfluous,
so deprecate it.

Resolves rdar://114092047

Author: tshortli

Sources/SwiftDriver/Jobs/FrontendJobHelpers.swift

@@ -107,15 +107,6 @@ extension Driver {
       commandLine.appendFlag("-aarch64-use-tbi")
     }
 
-    // Potentially unavailable enum cases are downgraded to a warning when building a
-    // swiftmodule, to allow building a module (or module interface) for an older
-    // deployment target than the framework itself.
-    if isFrontendArgSupported(.warnOnPotentiallyUnavailableEnumCase) {
-      if compilerOutputType == .swiftModule {
-        commandLine.appendFlag(.warnOnPotentiallyUnavailableEnumCase)
-      }
-    }
-
     // Enable or disable ObjC interop appropriately for the platform
     if targetTriple.isDarwin {
       commandLine.appendFlag(.enableObjcInterop)

Sources/SwiftOptions/Options.swift

@@ -789,7 +789,7 @@ extension Option {
   public static let warnLongFunctionBodiesEQ: Option = Option("-warn-long-function-bodies=", .joined, alias: Option.warnLongFunctionBodies, attributes: [.helpHidden, .frontend, .noDriver])
   public static let warnLongFunctionBodies: Option = Option("-warn-long-function-bodies", .separate, attributes: [.helpHidden, .frontend, .noDriver], metaVar: "<n>", helpText: "Warns when type-checking a function takes longer than <n> ms")
   public static let warnOnEditorPlaceholder: Option = Option("-warn-on-editor-placeholder", .flag, attributes: [.helpHidden, .frontend, .noDriver], helpText: "Downgrade the editor placeholder error to a warning")
-  public static let warnOnPotentiallyUnavailableEnumCase: Option = Option("-warn-on-potentially-unavailable-enum-case", .flag, attributes: [.helpHidden, .frontend, .noDriver], helpText: "Downgrade potential unavailability of enum case to a warning")
+  public static let warnOnPotentiallyUnavailableEnumCase: Option = Option("-warn-on-potentially-unavailable-enum-case", .flag, attributes: [.helpHidden, .frontend, .noDriver], helpText: "Deprecated, has no effect")
   public static let warnRedundantRequirements: Option = Option("-warn-redundant-requirements", .flag, attributes: [.helpHidden, .frontend, .noDriver], helpText: "Emit warnings for redundant requirements in generic signatures")
   public static let warnSwift3ObjcInferenceComplete: Option = Option("-warn-swift3-objc-inference-complete", .flag, attributes: [.frontend, .doesNotAffectIncrementalBuild], helpText: "Warn about deprecated @objc inference in Swift 3 for every declaration that will no longer be inferred as @objc in Swift 4")
   public static let warnSwift3ObjcInferenceMinimal: Option = Option("-warn-swift3-objc-inference-minimal", .flag, attributes: [.frontend, .doesNotAffectIncrementalBuild], helpText: "Warn about deprecated @objc inference in Swift 3 based on direct uses of the Objective-C entrypoint")